1. Field of the Invention
The present invention relates to a wiring structure and a method for fabricating the same, more particularly, to a copper (Cu) based wiring structure and a method for fabricating the same.
2. Related Art
In a display unit such as liquid crystal display, a lot of electronic devices are used. Further, as a semiconductor composing the electronic device, silicon (Si) is principally used. Herein, a bonding electrode contacting to Si is required to have a function of suppressing diffusion of the electrode material composing the bonding electrode into Si in a fabrication process of the electronic device including a heating process.
As a wiring material for an electrode, a wiring layer, a terminal electrode and the like used in the display unit, Japanese Patent Laid-Open No. 2007-72428 (JP-A 2007-72428) discloses the use of a copper alloy doped with an element having a smaller free energy of oxide formation than a free energy of oxide formation of copper (Cu), and having a diffusion coefficient in Cu than a self-diffusion coefficient of Cu (referred as “doping element” in the background of the Invention).
According to the wiring material disclosed by JP-A 2007-72428, the doping element moves to an interface between a substrate having a SiO2 film on its surface and the copper alloy and oxidizes the interface, so that an oxide layer comprising an oxide of the doping element is formed, thereby suppressing diffusion of copper to the substrate comprising silicon by means of the oxide layer (corresponding to a barrier layer which suppresses the diffusion of copper to the substrate comprising silicon).
However, the wiring material as claimed by JP-A 2007-72428 uses the oxide of Cu and the doping element doped to Cu as the barrier layer for suppressing the diffusion of the element to the silicon substrate. Therefore, it is required to securely provide an electrical conduction between the silicon substrate and the wiring provided at an opposite side with respect to the silicon substrate via the barrier layer comprising the oxide having an insulative property. For this case, it is necessary to provide the electrical conduction by a tunnel current by forming an extremely thin film barrier layer, in order to provide the electrical conduction of a current flow between the silicon substrate and the barrier layer. In addition, the barrier layer needs a certain thickness so that the barrier layer shows a function as the barrier layer.
Accordingly, in the wiring material disclosed by JP-A 2007-72428, a precise film thickness control of the barrier layer is required so as to achieve a balance between the function as the barrier layer and the electrical conduction between the silicon substrate and the wiring layer.
Furthermore, an oxide phase is occasionally formed in an oxidation process of the doping element at a grain boundary in the doping element. In this case, there is a possibility of generating a pinhole due to melting of the oxide phase, when an etching process is included in the fabrication process of the electronic device.